A doctrine' is a set of beliefs or principles that guide the behavior of an individual or group. The term 'ISM' is a three-letter suffix that is often used to refer to a doctrine or set of beliefs. For example, Marxism is a doctrine that is based on the writings of Karl Marx and Friedrich Engels, while capitalism is a doctrine that promotes private ownership of the means of production. Both of these doctrines have been influential in shaping the modern world.